Trying to get some attention for this bug again. Here's the short and sweet:
1. New Track with external MIDI routing
2. New MIDI Item
3. A note event
4. Playing the item and stopping will correctly cause a note-off to be sent
5. Playing the item and seeking will NOT send a note-off
Yes, there's a workaround (ReaControlMIDI), but concern troll that I am, the workaround is unintuitive and undocumented and until users discover it, the use case of external MIDI hardware is simply broken in a fairly basic way.
Please consider fixing this, I keep running into it (I have a lot of external MIDI hardware), despite various attempts to automate the workaround. Thank you!